Iran is pushing for as many concessions as it can get from the United States as nuclear talks enter the 11th hour before Tuesday's deadline, with negotiators digging in their heels on those issues.

"Sanctions and research and development are the areas where Iran and the G5+1 still have differences," a source close to the Iranian diplomatic team told Iranian state-sponsored Fars News on Sunday.

Iranian Foreign Minister Mohammad Javad Zarif wrote on his Facebook pageSunday that Tehran and the G5+1 negotiators had made "progress in reaching acceptable solutions, but we still have to work on some important issues; the key to striking an agreement lies in this strategic choice that the other side should make: pressure and sanctions or interaction and agreement by the other side."

Both sides have been involved in difficult negotiations, he said, and the arrival of G5+1 foreign ministers at the negotiations has complicated the situation.

On Saturday, Zarif, who is Iran's top negotiator in the nuclear talks, told Fars News that he thinks there can be a solution reached through a "common understanding," and that he has had good meetings with foreign ministers from France and Germany.

"I think our German and French friends are willing to play a very direct role in both the settlement [of the nuclear issue] and in future relations," he said. "We discussed very good points on how to proceed and resolve the problems."

"We have always said that achieving a solution is possible with political will; from my meetings with German and French foreign ministers I realized that the other side has also entered the talks with the same interest," he continued.

Sources familiar with the United States' stance confirmed the sticking points with The Washington Free Beacon, saying the added pressure from Iran may force more concessions.

"The Iranians are again outplaying the Americans," said of the sources from Europe. "They know they'll have to give up certain things eventually. So, they're digging in their heels on issues that mean everything and preparing to give ground on relatively minor issues — but not yet, and not until they see how much more the Americans are willing to give."

The Iranians are sensing weakness after other concessions already made by the United States on weapons, advanced centrifuges and sanctions, said an American source, adding that Iran is seeking sanctions relief "without dismantling anything."

Zarif tweeted Saturday that both sides must be flexible:

In negotiations, both sides must show flexibility.We have, and are ready to make a good deal for all.We await our counterparts' readiness.

A meeting set between Zarif and other foreign ministers was cancelled after reports he did not plan to appear, U.S. officials said.

Iran's diplomats are also pushing back on reports saying they may ship Tehran's enriched uranium to Russia, with one team member telling Fars News that "releasing such reports by the Western media is in line with media hype to disrupt the trend of the negotiations, which is, of course, a futile attempt."